Comet Likely Caused Dinosaur Extinction Not Asteroid, New Research Finds

A large comet was the likely cause of the end-Cretaceous mass extinction event that killed off the dinosaurs, not an asteroid, new research has found. The research found that the 180km-wide Chicxulub crater in Mexico was created by a smaller object than previously thought, and that when all the factors are taken together, an extremely fast traveling comet fits the evidence much better than an asteroid does.

“The overall aim of our project is to better characterise the impactor that produced the crater in the Yucatan peninsula [in Mexico],” said Jason Moore, from Dartmouth College in New Hampshire, in an interview with BBC News.

Previously, some of the primary evidence that helped to originally determine an impact event at the time of the dinosaur extinction, was the presence of a layer of sediments enriched in the chemical element iridium, in concentrations much higher than is naturally occurring, all around the globe. The only known mechanism for such an occurrence is an impact event from an asteroid or comet.

In the new work though, the researchers suggest that the “frequently quoted iridium values are incorrect. Using a comparison with another extraterrestrial element deposited in the impact – osmium – they were able to deduce that the collision deposited less debris than has previously been supposed. The recalculated iridium value suggests a smaller body hit the Earth. So for the second part of their work, the researchers took the new figure and attempted to reconcile it with the known physical properties of the Chicxulub impact.”

“For this smaller space rock to have produced a 180km-wide crater, it must have been travelling relatively quickly. The team found that a long-period comet fitted the bill much better than other possible candidates.”

“You’d need an asteroid of about 5km diameter to contribute that much iridium and osmium. But an asteroid that size would not make a 200km-diameter crater,” said Dr Moore.

“So we said: how do we get something that has enough energy to generate that size of crater, but has much less rocky material? That brings us to comets.”

Prof Mukul Sharma, also from Dartmouth College, added to that, saying: “You would need some special pleading for an asteroid moving very rapidly – although it is possible. But of the comets and asteroids we have looked at in the skies, the comets are the ones that are moving very rapidly.”

Long-period comets, such as this year’s Comet ISON and Comet PANSTARRS, are conglomerations of rock, ice, dust, and frozen gases, that follow highly eccentric orbits around the Sun. It takes them anywhere from hundreds, to thousands, to millions of years, just to make one orbit. Traveling all the way from the edges of the solar system, far beyond Pluto, to the inner solar system where we live.

“The extinction event 65 million years ago is now widely associated with the space impact at Chicxulub. It killed off about 70% of all species on Earth in just a short period of time, most notably the non-avian dinosaurs. The enormous collision would have triggered fires, earthquakes and huge tsunamis. The dust and gas thrown up into the atmosphere would have depressed global temperatures for several years.”

The new research already has some critics though, such as Dr. Gareth Collins, from the Imperial College London, who said: “I don’t think it is possible to accurately determine the impactor size from geochemistry.”

“Geochemistry tells you – quite accurately – only the mass of meteoritic material that is distributed globally, not the total mass of the impactor. To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater.”

He continued: “The authors suggest that 75% of the impactor mass is distributed globally, and hence arrive at quite a small-sized impactor, but in reality this fraction could be lower than 20%.”

If the percentage is on the lower end, than a far larger, slower-moving asteroid is still a possibility.

The authors of the new study have responded to this criticism by citing “recent studies suggesting mass loss for the Chicxulub impact was between 11% and 25%.” Which would mean 75-89% of the mass was distributed across the planet, and reinforce their theory of it being a comet impact.

“In recent years, several space objects have taken astronomers by surprise, serving as a reminder that our cosmic neighbourhood remains a busy place. On 15 February this year, 2012 DA14 – an asteroid as large as an Olympic swimming pool – raced past the Earth at a distance of just 27,700km (17,200mi). It had only been discovered the previous year.”

And of course there was an actual asteroid impact that same day, from an object estimated to be 17 meters long. That asteroid exploded in the atmosphere over Russia’s Ural mountains, not actually making landfall. The explosion released the energy of about 440 kilo tonnes of TNT though, so good thing it didn’t actually hit the ground. Around 1,000 people were still injured by it though, as a result of the the shockwave blowing out windows.

“Some 95% of the near-Earth objects larger than 1km have been discovered. However, only about 10% of the 13,000 – 20,000 asteroids above the size of 140m are being tracked.”

“There are probably many more comets than near-Earth asteroids, but NASA points out they spend almost all of their lifetimes at great distances from the Sun and Earth, so that they contribute only about 10% to the census of larger objects that have struck the Earth.”

With the speed that comets travel at, it’s easy to see how a comet impact could be that devastating. It’s an interesting thought though, something that lives far at the edges of the solar system periodically visiting the inner solar system and causing destruction.

About the Author

James Ayre 's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy. You can follow his work on Google+.

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